1. Field of the Invention
This invention relates to a radiation image storage panel, and more particularly to a radiation image storage panel comprising a support, a phosphor layer which comprises a binder and a stimulable phosphor dispersed therein and a light-reflecting layer containing a white pigment provided between the support and the phosphor layer.
2. DESCRIPTION OF PRIOR ARTS
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photoelectrically converting the emitted light to electric signals; and reproducing the electric signals as a visible image on a recording material such as a photosensitive film or on a displaying device such as CRT.
In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of using the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (stimulated emission) when excited with stimulating rays after having been exposed to a radiation such as X-rays. In the radiation image recording and reproducing method, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and the radiation image of the object is recorded on the radiation image storage panel in the form of a radiation energy-stored image (latent image). The radiation energy-stored image can be released as stimulated emission (light emission) by applying stimulating rays to the panel, for instance, by scanning the panel with stimulating rays. The stimulated emission is then photoelectrically converted to electric signals, so as to produce a visible image from the radiation energy-stored image.
It is desired for the radiation image storage panel employed in the radiation image recording and reproducing method to have a high sensitivity and to provide an image of high quality (high sharpness, high graininess, etc.).
For enhancing the sensitivity of a radiation image storage panel, it has been known that a light-reflecting layer is provided between the support and the phosphor layer, for instance, by coating a dispersion comprising a binder and a white pigment on the support to form a light-reflecting layer and subsequently forming the phosphor layer on the light-reflecting layer. A radiation image storage panel having the light-reflecting layer containing a white pigment is disclosed in Japanese Patent Provisional Publication No. 56(1981)-12600 (corresponding to U.S. Pat. No. 4,380,702), in which titanium dioxide, white lead, zinc sulfide, aluminum oxide and magnesium dioxide are mentioned as examples of the employable white pigment.
As a stimulable phosphor employable for the radiation image storage panel, there has been proposed a divalent europium activated alkaline earth metal fluorohalide phosphor, which has been thought to be a particularly preferable phosphor from the viewpoint of the luminance of stimulated emission, etc. This phosphor shows a band spectrum of stimulated emission in the near ultraviolet to blue region with the emission peak at approx. 390 nm.
The white pigments other than magnesium oxide disclosed in the above-mentioned Japanese Patent Provisional Publication No. 56(1981)-12600 show considerably low reflectance in the near ultraviolet region, though which show high reflectance in the visible region. Accordingly, especially when a stimulable phosphor which emits light in the near ultraviolet region as well as the visible region (for instance, the divalent europium activated alkaline earth metal fluorohalide phosphor shows an emission intensity in the near ultraviolet region higher than that in the visible region) is employed for the radiation image storage panel, the light-reflecting layer containing the one of the above-mentioned white pigments other than magnesium oxide does not show sufficiently high reflection characteristics and the sensitivity of the panel is not enhanced to a satisfactory level.
Among the white pigments disclosed in the aforementioned Publication, titanium dioxide is industrially prepared by the sulfate process (Norway Method) or the chloride process, while magnesium oxide is industrially prepared by calcining magnesium carbonate or magnesium hydroxide. Thus prepared white pigments are in the form of particles having small size, usually not more than 1 .mu.m. A pigment having such a small particle size is poor in dispersibility in a binder solution for the formation of light-reflecting layer, and the surface of the resulting light-reflecting layer tends to show poor smoothness, owing to the aggregation of the pigment particles on the surface of the layer. Such a light-reflecting layer having poor smoothness brings about difficulty in the formation of a phosphor layer with an even thickness thereon.